The present invention relates to brake sensors and, more particularly, the present invention relates to a brake shoe proximity sensor for drum brakes of a land vehicle.
Owners and operators of vehicles having drum brakes need to know the brake""s state of adjustment and brake wear to ensure safe operation of a vehicle. Brakes that are out of adjustment may not supply a sufficient amount of stopping power or may drag causing unnecessary wear and may cause the brakes to overheat. Brakes that are worn beyond an acceptable level may not supply a sufficient amount of stopping power for safe operation of the vehicle.
Some drum brakes are actuated by an S-cam or Z-cam assembly. S-cam brakes are actuated by a pneumatically driven push rod, which rotates a cam shaft on which the S-cam is mounted. The S-cam forces a pair of brake shoes against an interior surface of a brake drum. It is known in the art to measure the angular rotation of the S-cam to determine the adjustment of the brakes and wear of the brakes. However, to employ this method calculations must be performed to convert the angular rotation of the S-cam to an estimated travel distance at the brake shoe center. This calculation differs depending on the brake size and type, as well as the effective radius of the S-cam. For example, a different calculation is required for brakes having different diameters, when brakes use a single anchor pin rather than dual anchor pins, and when the effective radius of the cam changes. The computer or system reading the output of the S-cam type sensors must be configured to calculate the brake shoe travel of the particular brake type and size that it is monitoring.
It is known in the art to use an actuator position sensor to measure brake wear and running clearance adjustment of disc aircraft brakes. Brake wear and running clearance measurements are obtained by analyzing an output of position sensing circuit. Brake disc stack height is measured using actuator position sensors. The output of an actuator position sensor is compared by a controller to a reference brake disc stack height to provide an indication of an amount of brake wear. The displacement measured by the sensor can be used to determine a running clearance position of a reciprocating ram by subtracting a predetermined clearance value from the present displacement value.
Drum brakes for land vehicles are subject to harsh conditions. For example, drum brakes, which are continuously used, become very hot. In addition, drum brakes for land vehicles are exposed to dirt, debris and often salt.
What is needed is a drum braking system having a brake shoe proximity sensor that can be used to measure brake adjustment and brake lining wear on a variety of brake types and sizes without needing to be adjusted for the particular brake type and size being used, and which will hold up under the harsh environment in which drum brakes for land vehicles are operated.
The present invention concerns a brake system that utilizes a proximity sensor. The system includes a brake shoe that has a pad mounting surface. A brake lining pad is connected to the pad mounting surface of the brake shoe. The brake pad and brake shoe are concentric with a cylindrical surface of a brake drum. The brake shoe and pad are movable between a disengaged position where the brake pad is spaced apart from the cylindrical surface of the brake drum and an engaged position where the brake pad is in forceable engagement with the cylindrical surface of the brake drum. A position sensor is coupled to the brake shoe for sensing a position of the brake shoe relative to the position sensor. The position of the brake shoe relative to the position sensor provides an indication of the condition of the brake system.
The position sensor may utilize radar for sensing the position of the brake shoe. Alternatively, the position sensor may be a linear variable differential transformer or potentiometric displacement transducer which is physically connected to the brake shoe. In one embodiment, a brake spider is operably connected to the brake shoe facilitating movement of the shoe and pad between a disengaged position where the pad is spaced apart from the brake drum and an engaged position where the pad is in forceable engagement with the brake drum. A dust shield is connected to the brake spider. The dust shield has a first side that is oriented toward the brake shoe and a second side that is oriented away from the brake shoe. The dust shield includes an opening for a linkage in one embodiment.
In one embodiment, an insulation barrier is mounted to the second side of the dust shield. The position sensor is mounted to the insulation barrier. A protective coating or a protective cover may be disposed over the sensor. In one illustrated embodiment, the sensor is connected to the brake shoe by a linkage which extends through the opening in the dust shield. The linkage includes a first end that is connected to the position sensor and a second end that is connected to the brake shoe.
The position of a brake shoe is sensed by mounting the proximity sensor to a brake assembly. In one embodiment, the position of a brake shoe is sensed by mounting the proximity sensor to a brake spider backing plate or to a dust shield of the brake assembly. The input of the proximity sensor is coupled to the brake shoe. The proximity sensor detects motion of the brake shoe relative to the proximity sensor and provides an output indicative of the position of the brake shoe.
The proximity sensor of the brake system of the present invention can be used on a variety of sizes and types of brakes, without requiring modification to accurately calculate brake shoe motion for different sizes and types of brakes. In addition, the brake system with an integral proximity sensor constructed in accordance with the present invention is capable of operating when the brakes are at high temperatures for long periods of time and in the harsh environments in which drum brakes for land vehicles operate.
Additional features of the invention will become apparent and a fuller understanding obtained by reading the following detailed description in connection with the accompanying drawings.